1. Field of the Invention
This invention relates to electronic calipers and more particularly, to electronic calipers employing inductively coupled transducer elements.
2. Description of Related Art
Electronic calipers are common in manufacturing industries for measuring the thickness or other physical dimensions of an object. The principal component of these electronic calipers is almost universally a capacitive position transducer.
Capacitive transducers draw very little current. Therefore, capacitive transducers are well suited for use in battery-powered measurement tools, such as electronic calipers. Capacitive transducers operate under a parallel plate capacitor model. Within the capacitive transducer, a transmitter electrode and a receiver electrode are mounted on or in a slide. The transmitter electrode is connected to appropriate signal generating circuitry. The receiver electrode is connected to appropriate read circuitry.
The slide moves along a stationary scale. The scale includes a plurality of spaced-apart signal electrodes, which extend along the length of the scale. As the slide moves relative to the scale, the transmitter and receiver electrodes on the slide capacitively couple to the signal electrodes on the scale. The read circuitry determines the movement or position of the slide relative to the scale by comparing the phase of at least one signal coupled to a receiver electrode with the phase of at least one signal coupled to a transmitter electrode.
The capacitive position transducer may be an incremental type transducer or may be an absolute position type transducer. In the incremental type capacitive position transducer, the read circuitry provides only an indication of relative movement from a known point. In the absolute position type capacitive position transducer, the read circuitry provides an indication of the absolute position between the slide and scale. Incremental and absolute position type position transducers are disclosed in U.S. Pat. Nos. 4,420,754 and 4,879,508.
These capacitive position transducers are suitable when used in dry, relatively clean environments, such as in inspection rooms or engineering offices. However, these capacitive position transducers are desirably usable in calipers to measure dimensions in machine shops, construction sites and other relatively contamination-filled environments. In these environments, capacitive calipers can become contaminated by particulate matter and fluids, such as metal particles, grinding dust, and cooling or cutting fluids. The liquid or particulate contaminants find their way between the signal electrodes on the scale and the transmitter and/or receiver electrodes on the slide. The contaminants alter the capacitance between the signal electrodes and the transmitter and/or receiver electrodes in a manner unrelated to the position of the slide relative to the scale. In general, contaminants between the signal electrodes and the transmitter and/or receiver electrodes of a capacitive position transducer cause measurement errors through three different mechanisms. Primarily, the particulate or liquid may have a dielectric constant different from the dielectric constant of air. In this case, the capacitance between the signal electrodes and the transmitter/receiver electrodes sandwiching the contaminant will be greater than the capacitance between other ones of the signal and transmitter/receiver electrodes having the same relative geometry which do not have contaminants between them. As a result, the caliper will not provide an accurate indication of the position of the slide relative to the scale.
Secondarily, the contaminants may have a relatively high conductivity. Normally, the signal and transmitter/ receiver electrodes form an open circuit, such that no current flows between them. A conducive contaminant between the signal and transmitter or receiver electrodes closes this circuit. In particular, an RC circuit is formed, the contaminant forming the resistive element. The time constant of the RC circuit thus formed is a function of both the conductivity of the contaminant and the capacitance between the signal electrode and the transmitter and/or receiver electrodes. When the time constant is relatively short, the amplitude of the signal may decay so rapidly that the conventional circuitry employed in capacitive position transducers cannot properly sense the signal.
Thirdly, electrically conductive particles between the signal electrode and the transmitter and/or receiver electrodes may alter the field extending between the signal electrode and the transmitter and/or receiver electrodes which changes the capacitance between the signal electrode and the transmitter and/or receiver electrodes. Distortions in the electric field may also cause the signals between the signal electrode and the transmitter and/or receiver electrodes to be distorted such that the caliper circuitry does not provide an accurate indication of the position of the slide relative to the scale.
U.S. Pat. No. 5,172,485 to Gerhard et al. describes one approach to minimizing the adverse effects of contaminants in capacitive position transducers. This approach comprises coating the electrodes with a thin layer of dielectric material. The slide is then mounted on the scale so that the dielectric coating on the slide (transmitter and receiver) electrodes is positioned adjacent to the dielectric coating on the scale (signal) electrodes. That is, placing the dielectric coatings between the signal electrodes and the transmitter and receiver electrodes minimizes these adverse effects. In addition, the dielectric coating on the slide slidingly contacts the dielectric coating on the scale. The sliding contact between the dielectric coatings reduces the gap between the slide and the scale into which the contaminants intrude.
The sliding contact approach requires that the electrodes be resiliently biased toward each other. The resilient bias allows deviations from exact surface flatness and alignment to be accommodated by permitting the electrodes to move apart from each other. This allows the dielectric layers to be forced apart from each other. Thus, when such a capacitive position transducer is used in a highly contaminated environment, contaminants can force the slide away from the scale and collect between the slide and scale. Thus, this approach has not proven to be adequate under some circumstances.
However, using thick dielectric coats, rather than the thin coats taught by Gerhard et al. has reduced, to some extent, the negative effects due to contaminants collecting between the slide and scale. The thick dielectric coats create a pair of capacitors that are connected in series with the capacitance created by the contaminants. Since the capacitance created by the dielectric coats does not vary as the slide moves along the scale, the change in capacitance between the signal electrodes and the transmitter and/or receiver electrodes resulting from changes in the thickness or composition of the contaminants is dominated by the fixed capacitances created by the thick dielectric coats. Although using thick dielectric coats can reduce the problem caused by dielectric contaminants, this approach cannot completely eliminate the problem.
Another approach isolates the electrodes from the liquid and particulate contaminants. For example, the capacitive position transducer caliper may be sealed. However, sealing the caliper increases the fabrication and assembly costs and is often unreliable. Also, such seals are difficult to practically apply to all sizes and applications of electronic calipers.
Magnetic transducers are alternative types of position measuring transducers. Magnetic transducers are relatively insensitive to contamination caused by oil, water and other fluids. Magnetic transducers, such as the Sony Magnescale encoders, employ a read head that detects magnetic fields and a ferromagnetic scale that is selectively magnetized with one or more periodic magnetic patterns. The read head senses changes in the magnetic field as the read head moves relative to magnetic scale patterns on the scale. However, magnetic transducers themselves are affected by small particles, particularly ferromagnetic particles attracted to the magnetized scale. Consequently, magnetic transducers must also be sealed, encapsulated or otherwise protected to keep contaminants from affecting their accuracy. Magnetic transducers also do not offer the very low power consumption desired for electronic calipers. As a result, magnetic transducers are not used generally in calipers.
Inductive transducers, in contrast to both capacitive and magnetic transducers, are highly insensitive to cutting oil, water or other fluids as well as to dust, ferromagnetic particles, and other contaminants. Inductive transducers, such as the INDUCTOSYN.RTM. type transducers, employ multiple windings on one member to transmit a varying magnetic field received by similar windings on another member. The multiple windings can be a series of parallel hairpin turns repeated on a printed circuit board. An alternating current flowing in the windings of the first member generates the varying magnetic field. The signal received by the second member varies periodically based on the relative position between the two members. A position determining circuit connected to the varying signal from the second member can determine the relative position between the first and second members. However, both members are active. Therefore, each member must be electrically coupled to the appropriate driving circuitry, which increases manufacturing and installation costs. Additionally, because inductive transducers require both members to be electrically coupled, inductive transducers are difficult to incorporate into hand-held devices, such as calipers.
Other motion or position transducers that are insensitive to contaminants, yet which can be more inexpensively manufactured than capacitive, magnetic or inductive transducers, are described in U.S. Pat. Nos. 4,697,144 to Howbrook, 5,233,294 to Dreoni, and 4,743,786 to Ichikawa et al., and British Patent Application 2,064,125 to Thatcher. These references disclose position detection devices that sense position between an energized member and an inactive or unenergized member. The transducing systems described in these references eliminate electrical intercoupling between the two moving members, a drawback of inductive transducers. However, these systems generally fail to provide the high accuracy of inductive or capacitive transducers.
Additionally, in some of these transducing systems, the inactive member is preferably ferromagnetic, which produces a strong magnetic field. Alternately, the inactive member is moved within a magnetic field defined and concentrated by a complex structure formed in or on the active member. Additionally, none of these systems provide the combination of low power operation and sufficient accuracy and measuring range which users demand of calipers. The transducing systems disclosed in these references also produce output signals which are discontinuous or which are not a simply-prescribed function of position. Such signals contribute to inaccurately determined relative positions over extended distances. Furthermore, the transducing systems disclosed in these references are in other ways poorly suited when incorporated into a caliper.